1. Field of the Invention
The present invention relates to a propylene block copolymer. More particularly, it relates to a propylene block copolymer excellent in transparency, impact resistance at a low temperature, and bleeding control effect at a heat treatment.
2. Description of the Related Arts
Polypropylene is excellent in heat resistance, rigidity and the like, and hence has been used widely in fields of a film, sheet, container and the like.
In recent years, a material having flexibility, transparency, impact resistance and heat resistance all together has been required. However, a propylene homopolymer is excellent in heat resistance, but it is inferior in flexibility, transparency and impact resistance at a low temperature. A random copolymer of propylene with another .alpha.-olefin is excellent in transparency, but has insufficient impact resistance at a low temperature which limits the use thereof.
As a method for imparting the impact resistance thereto, there have been made some attempts for improvement using a propylene block copolymer mainly.
As a block copolymer composed of a polypropylene portion (herein-after, abbreviated as P) and ethylene-propylene random copolymer portion (herein-after, abbreviated as EP), JP-A-06-093061 discloses a P-EP block copolymer obtained by producing a polypropylene portion mainly composed of propylene with a Ziegler-Natta catalyst, substantially in the absence of an inert solvent in the first step, and subsequently producing an EP copolymer portion in a vapor phase in the second step. The polymerization in the vapor phase gives a polymer superior in impact resistance at a low temperature as compared with a conventional polymer produced in a solution polymerization method, however, the transparency of the polymer produced in the vapor phase is insufficiently low.
There is also known a block copolymer composed of an EP copolymer portion and the other EP copolymer portion different in the composition. JP-A-56-084712 discloses a propylene block copolymer having a melt flow index of 0.01 to 0.3 g/10 minutes, and being composed of a portion A (EP copolymer portion obtained in the first step) having an ethylene content of less than 20 wt % and a portion B (EP copolymer portion obtained in the second step) having an ethylene content of 20 wt % or more. Concretely, the polymer prepared by a solution polymerization method in the working example, is disclosed, but it was unsatisfactory in transparency. Also, in the comparative example, it is disclosed that a polymer in which the ethylene concentration of the portion B is reduced to improve transparency, however, the impact strength is unsatisfactorily low.
In JP-A-08-283491, there is disclosed a composition obtained by blending a random copolymer of ethylene, propylene and .alpha.-olefin with a random copolymer of ethylene, propylene and .alpha.-olefin having different comonomer contents therefrom. However, in the case of blend, a homogeneous mixing as in the block copolymer can not be attained, the blend had an insufficient transparency.
The present inventors has studied in order to overcome the deficiencies of conventional block copolymers, and develop a propylene block copolymer with good transparency and improved impact resistance at a low temperature. As results, they proposed a method for producing a composition having transparency, impact resistance and the like all together, by producing an EP-EP type block copolymer with a specific composition substantially in the absence of an inert solvent.
However, the use of the specific EP--EP type block copolymer obtained by polymerization substantially in the absence of the solvent is unsatisfactory in the applications such as packing bag for medical care required to extremely reduce a change in transparency at the time of high-temperature heat treatment.
The change in transparency is caused mainly by a change in haze resulting from the bleeding mainly of a low molecular weight component by heat treatment. Accordingly, there has been required a resin composition whose bleeding at a high temperature is controlled.
It is noted that the block copolymer herein stated means the one obtained by sequentially conducting the polymerization of the first step, and the polymerization of the second step is not a true block copolymer, but is a kind of blend (composition).